The present invention relates to the general field of metal matrix composite materials. More particularly, it relates to a method and to a device for coating ceramic material fibers in metal by a liquid technique.
One of the fields of application of the invention is that of aviation, and more precisely that of turbojets in which having recourse to metal matrix composite materials enables significant weight savings to be achieved.
In known manner, metal matrix composite materials are constituted by a matrix based on a metal alloy that is reinforced by fibers, the fibers being made of ceramic, for example. Such materials present high level performance in terms of stiffness and strength and they can be used instead of monolithic alloys in order to make turbojet parts such as disks for compressors or turbines, shafts, actuator cylinders, etc.
In advantageous manner, metal matrix composite materials can also be used for providing local reinforcement that is put into place in monolithic alloy parts such as blades, casings, spacers, etc. Under such circumstances, the reinforcement is generally fabricated from a half-finished product referred to as “coated fiber” that is constituted by a ceramic central core that is coated in a metal sheath.
The ceramic core of such coated fibers may be coated using a vapor technique in an electric field, e.g. by electrophoresis, or by coating using a liquid technique in a bath of liquid metal. For this purpose, document EP 0 931 846 describes a method of coating ceramic material fibers in metal by a liquid technique. That method consists essentially in maintaining a charge of molten metal in levitation inside a crucible, and in causing a tensioned fiber of ceramic material to travel through said charge. At the outlet from the bath of metal, the fiber is coated in a metal coating of thickness that depends in particular on the travel speed of the fiber.
In practice, it has been found that the quality of the coating obtained by that type of liquid coating method depends to a large extent on the instantaneous height of fiber that is immersed in the metal charge. As coating progresses, the weight of the charge decreases, thereby automatically reducing the instantaneous height of fiber that is immersed therein, if the position of the fiber in the crucible is kept constant. As a result, the thickness of the coating varies continuously along the coated fiber until the nominal conditions for coating are no longer satisfied, which means that coating must be interrupted. This means that for a given charge of metal, the quality of the resulting coating is acceptable over a limited length only of the fiber, which length is a function of the selected coating thickness. For example, for charges having a volume of 50 cubic centimeters (cm3) and for a coating of small thickness, the length of coated fiber that presents quality that is acceptable may be several hundreds of meters. In contrast, for a thicker coating (of the order of 50 micrometers (μm)), the length of coated fiber that is of acceptable quality is no more than a few tens of meters.
Consequently, the quality of the coated fiber that is obtained by a liquid coating method, even though it remains acceptable, is not optimized insofar as the thickness of the coating is not constant over the entire length of the coated fiber. Correspondingly, the productivity of that type of coating method is relatively low since the length of coated fiber is limited.
In order to solve that problem and to make coated fibers of great length, proposals have been made to refill the bath of molten metal while coating is taking place by using powders, straws, or fibers. Nevertheless, that solution presents the drawback of being relatively expensive since means for delivering fibers or powders are themselves expensive. Furthermore, incorporating new matter into a given bath can lead to instabilities that are harmful to the coating process.